Safety device for electric heaters



NOV. 17, 1936..v H` A, SOVERHlLL 2,061,258

SAFETY DEVICE FOR ELECTRIC HEATERS Filed July 6, 1935 V LA | lNVENTORPatented Nov. 17, 1936 PATENT OFFICE SAFETY DEVICE FOR ELECTRIC HEATERSHarveyA. Soverhill, Chicago, Ill., assignor to The American FoundryEquipment Company, Mishawaka, Ind., a corporation of DelawareApplication July 6, 1935, Serial No. 30,068

8 Claims.

The invention relates to electric heaters, and more particularly tosafety devices for preventing burning out of such heaters.

The invention relates more particularly to forced draft electric airheaters comprising a grid or other electric heating element forgenerating heata fan for forcing a stream of air through the grid and amotor for driving the fan. If the air stream should fail for any reason,the temperature of the electric grid would rise to such a temperature asto burn itself out if some safety means were not provided.

According to a preferred form of the invention, a safety device isprovided comprising a safety heating coil disposed in the current of airl generated by the fan. This coil is heated by an electric current, butso long as the forced draft of air remains normal and undiminished, thesafety device is kept comparatively cool. If the air draft should fail,the safety heating coil supplies sufficient heat to melt a fusiblematerial permitting a switch in the safety device to release, openingthe circuit and removing the electric energy from the h'eater grid.

'I'he invention also consists in certain new and original features ofconstruction and comlbinations of parts hereinafter set forth andclaimed.

Although the novel features which are believed to be characteristic ofthis invention will be particularly pointed out in the claims appendedhereto, the invention itself, as to its objects and advantages, and themanner in which it may be carried out, may be better understood byreferring to the following description taken in connection With theaccompanying drawing forming a part thereof, in which Fig. 1 is a rearelevation of one type of electric heater to which the safety device ofthe invention is applied;

Fig. 2 represents somewhat diagrammatically the electric diagram;

Fig. 3 is an open face view, with the cover removed, of the safetydevice;

Fig. 4 is a section on the line 4-4 of Fig. 3;

Fig. 5 is a perspective of.a slide member; and

Fig. 6 illustrates the manner in which the fusible material is utilizedIn the following description and in the claims, various details will beidentified by specific names for convenience, but they are intended tobe as generic in their application as the art will permit.

Like reference characters denote like parts in the several figures ofthe drawing.

In the drawing accompanying and forming (Cl. 21S-39) part of thisspecification, a certain specific disclosure of the invention is madefor purposes of explanation, but it will be understood that the detailsmay be modified in various respects without departure from the broadaspect of the 5 invention.

Referring now to the drawing, and more parlticularly to Fig. 1, theheater comprises, in general, a hollow casing I0 having a grid IImounted in the front of the casing, a fan I2 in back of the grid, amotor I3 in back of the fan and a service switch I4 and safety deviceI5, both mounted in back of the fan. For a detailed description andillustration of the structure of this heater, attention is directed tocopending 15 application Serial No. 30,067, filed July 6, 1935.

The grid II has a circular or cylindrical body 47 which has a pluralityof outwardly extending radial flanges 48 and a plurality of inwardlyextending flanges 49. incorporated therein one or m'ore individualresistance elements 40 through which electricity ows to generate heat(Fig. 2).

The details of construction of the casing I0, heat generating grid II,fan I2 and motor I3 are 25 immaterial so far as the safety control isconcerned. In the form shown, the fan I2 operates to draw cold airthrough the rear screen 24 of the casing, discharging this air throughthe grid II where -it becomes. heated and out through a 30 correspondingscreen in the front face of the casing. As illustrated especially inFig. 2, the safety device I5 is on the suction side of the fan I2.

Referring to Figs. 4 to 6, the safety device I5 .35 comprises aninsulating casing 55 having a removable cover 56. The safety device issecured on the inside of the casing I0 to the rear wall thereof with itsknob I5 projecting'through to the outside of the casing. Disposed withinthe 40 insulating casing 55 is an insulating body 60 on which the safetymechanism is mounted.

Secured to the insulating body is a heat conducting rod 3l surrounded bythe safety heating coil 36, both of these members being disposed in 45the air'stream on the suction side of the fan I2 (see Figs. 1 and 2).The rod 31 has an axle member 59 at its lower end on which is mounted aratchet wheel 51 with fusible material, such as a special form of solder58, therebetween. 50

The insulating knob 'I5 is slidably mounted within the casing 55 and isconnected to a com. pound slide 6| (Fig. 5) made up of a ratchet plate62 and a cam plate 63, these plates being slidably mounted within a slotin the insulating 55 The body 41 may have 20 body 60. 'I'he ratchetplate 62 has a. pawl 64 and an eye 65. The cam plate 63 has a camportion 16, a U-shaped portion 66 and an angle 61 se cured to the knob15. A coil spring 66 operates between lugs on the plates 62.and 63normally to hold the cam portion 16 away from the pawl 64.

The safety devicehas a pair of stationary contacts 10 cooperating with apair of movable contacts 69 mounted on the end of a forked arm 1I whoseends are disposed in a plate 12 secured to the body 60. The forked arm1I is.held in position by a coil spring 13 whose ends are securedrespectively to the arm 1I and plate 12. The spring 13 passes throughthe collar 14 disposed in the eye 65 and fork 66.

Referring now to Fig. 2, it will be noted that both heaters 40 are undercontrol of the service switch I4 and of the safety device I5. The safetycoil 36 is in series with one of the heater elements 40, it beingunderstood that the voltage drop through the coil 36 is substantiallynegligible.

In operation, so long as the air flow caused by the fan I2 is normal,the coil 36 and rod 31 are kept suillciently cool to prevent melting ofthe fusible material 58 in spite of the heat generated by the coil 36.AIf the air stream should fail either due to motor failure or fanfailure, or due to paper either totally or partially clogging the heatergrid I I or intake screen 24, the lack of cooling effect on coil 36 androd 31 will cause the temperature of rod 31 to increase suiciently tomelt the solder 58. This permits the ratchet wheel 51 to rotate,releasing the pawl 64 and permitting spring 13 to move the slide 6I andpull open the contacts 69 and 1I), breaking the circuit to both themotor I3 and heater elements 40. Thus, the current supply is brokenbefore the heater elements can overheat sumciently to damage themselves.

After the obstruction is removed or the defect otherwise overcome, theknob 15 is pushed inwardly, causing the pawl 64 to engage ratchet wheel51 which is now in xed position since the solder 56 has again hardened.It will be noted that the inward motion of knob 15 moves both plates 62and 63 inwardly together, with the cam portion 16 disengaged from pawl64.

If it is desired to use the safety device I5 as a service switch, it ,isonly necessary to pull out the knob 15. 'Ihis action moves plate 63 withrespect to plate 62, causing the cam portion 16 to engage pawl 64,removing the latter from the ratchet wheel 51, thus permitting thespring 13 to disengage the contacts 69 and 10.

It will be understood that the safety device may be placed anywhere inthe air stream, either in the air stream before it is subjected to theelectric heater element or after the air leaves the heater element. Thesize of the heater wire of coil 36, its length and the distance itprojects into the air stream will be governed by conditions, such as thevoltage and the kilowatt capacity of the heater. It will be furthermoreunderstood that with large capacity heaters requiring the use ofcontactors for controlling the current to the heater element 40, thecontacts 69 and 10 may be placed in series with the electromagnet on thecontactor.

Thus, a safety control has been provided which is simple in constructionand emcient in operation. It is self-contained and requires no directmechanical connection with the heater element. Yet it surely protectsthe electric heat- 2,061,258 .ing element from burning out since itrespomh instantly to failure of normal air supply. Bo long as the heateris receiving its normal air sup ply, the heating elements cannot burnout and the safety device is kept sumciently cool to prevent itsoperation. The connection of the safety coil 36 in series with one ofthe heater elements eliminates the necessity for an extra resistancewhich would otherwise be necessary to cut down the voltage on the coil36.

While certain novel features of the invention have been disclosed andare pointed out in the annexed claims, it will be understood thatvarious omissions, substitutions and changes may be made by thoseskilled in the art without departing from the spirit of the invention.

What is claimed is:

l, In an electric heater, a main heater element, means for establishinga current' of air across said main heater element, a small pilot heatersubjected to said air current, and means operated by said pilot heatergoverned by failure of said air current for removing electric power fromsaid main heater element.

2. In an electric heater, a main heater element, an electric fan forestablishing a current of air across said main heater element. a smallpilot heater, a heat responsive element controlling the supply ofelectric power to said main heater element, said heat responsive elementbeing subjected to the combined effects of said pilot heater and of saidair current.

3. In an electric heater control system, an electric main heaterelement, an electric fan for discharging a stream of air across saidheater element, a safety device comprising a safety heater coil, switchcontacts controlling the cur rent to said main heaterelement. a springnormally tending to open said contacts, a heat conducting rod withinsaid vheater coil, a ratchet wheel on said rod, fusible material betweensaid rod and ratchet wheel, a pawl connected to said contacts andengaging said ratchet wheel for resisting said spring, said coil and rodbeing lo cated in said air stream, said fusible material melting uponfailure of said air stream.

4. In an electric heater control system, an electric main heaterelement, an electric fan for discharging a blast of air across saidheater element, a safety device comprising a safety heater, switchcontacts controlling the heating current to said main heater element, aspring normally tending to open said contacts, a holding member forresisting said spring, said holding member being normally solid butbecoming liquid under abnormal temperatures, said holding member beingsubjected to heat from said safety heater, said safety device beinglocated in the path of the air stream passing through said heater, saidholding member becoming molten upon failure of said air stream, saidsafety heater being in series with said mai heater element.

5. In an electric heater contro system, an electric main heater element,an electric fan for discharging a blast of air across said heaterelement, a safety device comprising a safety heater. switch contactscontrolling the heating current in said main heater element. a springnormally tending to open said contacts, a holding member for resistingsaid spring, said holding member being normally solid but becomingliquid under abnormal temperatures, said holding member being subjectedto heat from said safety heater, said safety device being located in thepath of the air stream passing through said heater. said holding memberbecoming molten upon failure of said air stream.

6. In an electric heater control system, a casing, an electric mainheater element therein, an electric fan for blowing a blast of airacross said heater element also in said casing, a unitary safety devicecomprising a housing, switch contacts in said housing for controllingthe power supply to said main heater element, a spring normally tendingto open said contacts, a heat conducting rod, P, pilot heating coilaround said rod, a ratchet wheel on said rod, fusible material betweensaid rod and said ratchet wheel, a pawl connected to said contacts andengaging said ratchet wheel, said housing being secured to said casingwith said rod and coil in the air stream.

7. In a control system for an electric heater, an electric heaterelement, an electric fan discharging a stream of air through said heaterelement, a safety device in said air stream, said safety devicecomprising a plunger having a movable contact, cooperating stationarycontacts bridged by said movable contact, a safety spring normallytending to open said plunger, a ratchet wheel, a support for saidratchet wheel. a normally solid, meltable substance holding said ratchetwheel stationary, a heat conducting rod extending from said ratchetwheel into said air stream, a safety heating coil around said rod, saidsafety coil being in series with said main heater element, a pawl onsaid plunger engaging said ratchet wheel to keep the switch closed, ahand-engaging portion on said plunger and means whereby said contactsmay be opened by said hand-engaging portion, said meltable substancemelting upon failure of the air stream, thereby permitting said springto open said switch contacts.

8. In an electric heater, a main heater element, means for establishinga current of air across said main heater element, a small pilot heatersubjectedv to said air current, and means operated by said pilot heatergoverned by failure of said air current for removing electric powerfromsaid main heater element, said main heater element and small pilotheater being in series with the electric power.

HARVEY A. .SOVERHIIL

